[llvm] [VPlan] Return Max from computeCost without underlying instrs. (PR #109708)

[Florian Hahn via llvm-commits]

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@@ -312,15 +311,19 @@ InstructionCost VPRecipeBase::cost(ElementCount VF, VPCostContext &Ctx) {
 InstructionCost VPRecipeBase::computeCost(ElementCount VF,
                                           VPCostContext &Ctx) const {
   // Compute the cost for the recipe falling back to the legacy cost model using
-  // the underlying instruction. If there is no underlying instruction, returns
-  // 0.
+  // the underlying instruction. If there is no underlying instruction or the
+  // cost is computed by the recipe's computeCost, returns
+  // InstructionCost::getMax. It is used as  a sentinel value to detect recipes
+  // without underlying instructions for which no forced target instruction cost
+  // should be applied.
+
   Instruction *UI = getInstructionForCost(this);
   if (UI && isa<VPReplicateRecipe>(this)) {
     // VPReplicateRecipe may be cloned as part of an existing VPlan-to-VPlan
     // transform, avoid computing their cost multiple times for now.
     Ctx.SkipCostComputation.insert(UI);
   }
-  return UI ? Ctx.getLegacyCost(UI, VF) : 0;
+  return UI ? Ctx.getLegacyCost(UI, VF) : InstructionCost::getMax();
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fhahn wrote:

Adjusted to use getLegacyCost in a few relevant sub-classes (VPSingleDefRecipe/VPInterleaveRecipe). Should hopefully be clearer and simpler now

https://github.com/llvm/llvm-project/pull/109708